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Abstract:

A method for manufacturing a switching element which has enough
resistance to repeat switching operations and which can be miniaturized
and have low power consumption, and a display device including the
switching element are provided. The switching element includes a first
electrode to which a constant potential is applied, a second electrode
adjacent to the first electrode, and a third electrode over the first
electrode with a spacer layer formed of a piezoelectric material
interposed therebetween and provided across the second electrode such
that there is a gap between the second electrode and the third electrode.
A potential which is different from or approximately the same as a
potential of the first electrode is applied to the third electrode to
expand and contract the spacer layer, so that a contact state or a
noncontact state between the second electrode and the third electrode can
be selected.

Claims:

1. A method for manufacturing a switching element comprising: forming a
first conductive film and a second conductive film over a substrate;
forming a spacer layer so as to cover the first conductive film and the
second conductive film; selectively forming an opening in the spacer
layer to expose at least a part of the second conductive film; forming a
sacrificial layer over a part of the spacer layer and the exposed part of
the second conductive film; forming a third conductive film so as to
cover the sacrificial layer; and removing the sacrificial layer so that a
gap is formed between the second conductive film and the third conductive
film in the opening.

2. A method for manufacturing a switching element comprising: forming a
first conductive film and a second conductive film over a substrate;
forming a spacer layer so as to cover the first conductive film and the
second conductive film; selectively forming an opening in the spacer
layer to expose at least a part of the second conductive film; forming a
conductor in the opening; forming a third conductive film so as to cover
the spacer layer and the conductor; and making the conductor aggregate by
a heat treatment so that a gap is formed between the conductor and the
third conductive film.

3. The method for manufacturing a switching element according to claim 2,
wherein nanoparticles of silver (Ag) are used for the conductor.

4. The method for manufacturing a switching element according to claim 1,
wherein a piezoelectric material is used for the spacer layer.

5. The method for manufacturing a switching element according to claim 4,
wherein any one of a crystal (SiO2) film, a lead zirconate titanate
(PZT) film, a lithium niobate (LiNbO3) film, a barium titanate
(BaTiO3) film, a lead titanate (PbTiO3) film, a lead
metaniobate (PbNb2O6) film, and a zinc oxide (ZnO) film is used
as the piezoelectric material.

6. The method for manufacturing a switching element according to claim 2,
wherein a piezoelectric material is used for the spacer layer.

7. The method for manufacturing a switching element according to claim 6,
wherein any one of a crystal (SiO2) film, a lead zirconate titanate
(PZT) film, a lithium niobate (LiNbO3) film, a barium titanate
(BaTiO3) film, a lead titanate (PbTiO3) film, a lead
metaniobate (PbNb2O6) film, and a zinc oxide (ZnO) film is used
as the piezoelectric material.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of U.S. application Ser. No.
12/047,883, filed Mar. 13, 2008, now allowed, which claims the benefit of
a foreign priority application filed in Japan as Serial No. 2007-078558
on Mar. 26, 2007, both of which are incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a switching element, a method for
manufacturing a switching element, and a display device including a
switching element. In particular, the present invention relates to a
display device including a mechanical switch.

[0004] 2. Description of the Related Art

[0005] In recent years, as a display device using a liquid crystal panel
or the like, an active-driving display device using thin film transistors
(TFTs) is employed in many cases for higher definition. In the case of an
active-driving display device, the potential of each pixel electrode can
be independently controlled and thus there is no crosstalk such as a leak
of an electrical field to an adjacent pixel, as there is in the case of a
passive-driving display device; therefore, a panel with less unevenness
in display and a higher contrast ratio can be manufactured.

[0006] However, active driving has a problem in that because switching
on/off of a potential of a pixel electrode is performed electrically
using TFTs, an off current (leak current) flows even in an off state and
thus it is difficult to maintain a completely off state. When there is an
off current, it is difficult to hold a potential of a pixel electrode.
Therefore, it is necessary to provide an additional storage capacitor.
Further, the higher an off current is, the larger the storage capacitor
needs to be. Thus, problems such as lower driving frequency and a flicker
in a screen are caused. Further, because it is necessary to supply an
excess charge to the storage capacitor, power consumption increases.

[0007] In order to solve the above problems, a display device including a
mechanical switch as a switching element has been proposed (for example,
Cited Document 1: Japanese Published Patent Application No. 119-92909 and
Cited Document 2: Japanese Published Patent Application No. 2000-35591).
A mechanical switch is operated instead of a switching element such as a
transistor, by displacing a fixed flat spring by electrostatic force so
that a conductive film (upper electrode) provided at a top portion of the
flat spring and a conductive film (lower electrode) such as a pixel
electrode are or are not in contact with each other. Further, a display
device in which an upper electrode and a pixel electrode come or cease to
be in contact with each other depending on contraction of a piezoelectric
element has been proposed (for example, Cited Document 3: Japanese
Published Patent Application No. H11-174994).

[0008] However, in the case where a switching element is provided so as to
have a plate structure in which one side is fixed (Cited Document 1),
stress is locally concentrated at the time of switching operation of the
switching element (a contact or noncontact operation between an upper
electrode and a lower electrode); therefore, damage of the switching
element is a problem. Similarly, in the case where a flexible thin film
and a conductive film are bent with the use of a supporting board to
perform a contact or noncontact operation (Cited Document 2), stress is
also caused in a specific portion of the flexible thin film; therefore,
resistance of an element is a problem. Further, it is necessary to form a
somewhat large gap; therefore, it is difficult to miniaturize the
switching element.

[0009] As for the structure of a piezoelectric switch disclosed in Cited
Document 3, even when a switching element is off (an upper electrode and
a pixel electrode are not in contact with each other), a potential
difference is generated in a piezoelectric element as a signal line which
functions as a lower electrode changes, so the switching element cannot
be completely turned off, and therefore, malfunction might occur. In
particular, in a case where a gap between the upper electrode and the
pixel electrode is small, there is a significant possibility that
malfunction might occur.

[0010] Meanwhile, in the case where the gap is widened to prevent
malfunctions, in order that the switching element be turned on, it is
necessary for a potential difference between the upper electrode and the
lower electrode to be larger, and thus, power consumption of the
switching element increases. In this case, a potential difference between
a wiring connected to the upper electrode and a wiring connected to the
lower electrode is also larger; therefore, there is a possibility that a
leak current might occur in a portion where the wirings intersect with
each other. Consequently, a display defect might be caused due to
reduction in voltage applied to the piezoelectric element as well as
increase in power consumption.

SUMMARY OF THE INVENTION

[0011] In view of the above problems, an object of the present invention
is to provide a switching element which has enough resistance even in the
case where switching operations are repeated and which can be
miniaturized and have low power consumption, a method for manufacturing
the switching element, and a display device including the switching
element.

[0012] One switching element of the present invention includes a first
electrode to which a constant potential is applied, a second electrode
provided adjacent to the first electrode, and a third electrode provided
over the first electrode and across the second electrode such that there
is a gap between the second electrode and the third electrode. A spacer
layer formed of a piezoelectric material is between the first electrode
and the third electrode. A potential which is different from or
approximately the same as a potential of the first electrode is applied
to the third electrode to expand and contract the spacer layer, so that a
contact state or a noncontact state between the second electrode and the
third electrode can be selected.

[0013] According to the above-described switching element of the present
invention, in the above structure, a potential different from that of the
first electrode is applied to the third electrode so that the second
electrode and the third electrode come to be in a contact state, and a
potential approximately the same as that of the first electrode is
applied to the third electrode so that the second electrode and the third
electrode come to be in a noncontact state.

[0014] One display device of the present invention includes a first
electrode to which a constant potential is applied, a second electrode
provided adjacent to the first electrode, a third electrode provided over
the first electrode and across the second electrode such that there is a
gap between the second electrode and the third electrode, and a pixel
electrode electrically connected to the second electrode. A spacer layer
formed of a piezoelectric material is between the first electrode and the
third electrode. A potential which is different from or approximately the
same as a potential of the first electrode is applied to the third
electrode to expand and contract the spacer layer, so that a contact
state or a noncontact state between the second electrode and the third
electrode can be selected.

[0015] According to the above-described display device of the present
invention, in the above structure, a potential different from that of the
first electrode is applied to the third electrode so that the second
electrode and the third electrode come to be in a contact state, and a
potential approximately the same as that of the first electrode is
applied to the third electrode so that the second electrode and the third
electrode come to be in a noncontact state.

[0016] One method for manufacturing a switching element of the present
invention includes forming a first conductive film and a second
conductive film over a substrate; forming a spacer layer so as to cover
the first conductive film and the second conductive film; selectively
forming an opening in the spacer layer to expose at least part of the
second conductive film; forming a sacrificial layer over part of the
spacer layer and the second conductive film at least part of which is
exposed; forming a third conductive film so as to cover the sacrificial
layer; and removing the sacrificial layer so that a gap is formed between
the second conductive film and the third conductive film in the opening.

[0017] Another method for manufacturing a switching element of the present
invention includes forming a first conductive film and a second
conductive film over a substrate; forming a spacer layer so as to cover
the first conductive film and the second conductive film; selectively
forming an opening in the spacer layer to expose at least part of the
second conductive film; forming a conductor in the opening; forming a
third conductive film so as to cover the spacer layer and the conductor;
and making the conductor aggregate by a heat treatment so that a gap is
formed between the conductor and the third conductive film.

[0018] According to the present invention, a switching operation (contact
or noncontact operation) is performed by expansion and contraction of an
insulating film provided between conductive films, and thus, stress is
not concentrated in a portion of a switching element; therefore, the
element is prevented from being damaged. Further, since expansion and
contraction in up and down directions of the insulating film provided
between the conductive films are utilized, it is not necessary to widen a
gap 104, and therefore, the switching element can be miniaturized.
Further, according to the present invention, since a potential difference
between a conductive film which functions as an upper electrode and a
conductive film which functions as a lower electrode is constant when the
switching element is off, malfunctioning of the switching element can be
prevented even in a case where the switching element is driven with low
power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIGS. 1A to 1C are diagrams each showing a structure of a switching
element of a display device of the present invention.

[0020] FIGS. 2A and 2B are diagrams each showing a structure of a
switching element of a display device of the present invention.

[0021] FIGS. 3A and 3B are diagrams showing a driving method of a display
device of the present invention.

[0022] FIG. 4 is a diagram showing a structure of a display device of the
present invention.

[0023] FIGS. 5A and 5B are diagrams each showing a structure of a display
device of the present invention.

[0024] FIGS. 6A and 6B are diagrams each showing a structure of a display
device of the present invention.

[0025] FIGS. 7A to 7E are diagrams showing an example of a method for
manufacturing a display device of the present invention.

[0026] FIGS. 8A to 8D are diagrams showing an example of a method for
manufacturing a display device of the present invention.

[0027] FIGS. 9A to 9E are diagrams showing an example of a method for
manufacturing a display device of the present invention.

[0028] FIGS. 10A to 10E are diagrams showing an example of a method for
manufacturing a display device of the present invention.

[0029] FIGS. 11A and 11B are diagrams showing an example of a method for
manufacturing a display device of the present invention.

[0030] FIGS. 12A to 12D are diagrams showing an example of a method for
manufacturing a display device of the present invention.

[0031] FIGS. 13A to 13C are diagrams showing an example of a method for
manufacturing a display device of the present invention.

[0032] FIGS. 14A and 14B are diagrams showing an example of a method for
manufacturing a display device of the present invention.

[0033] FIGS. 15A to 15C are diagrams each showing a structure of a
switching element of a display device of the present invention.

[0034]FIG. 16 is a diagram showing a driving method of a display device
of the present invention.

[0035] FIGS. 17A to 17H are diagrams each showing an application mode of a
display device of the present invention.

[0037] Hereinafter, embodiment modes of the present invention will be
described with reference to the accompanying drawings. However, the
present invention can be carried out in many different modes, and it will
be easily understood by those skilled in the art that various changes and
modifications can be made to the modes and their details without
departing from the spirit and scope of the invention. Therefore, the
present invention should not be construed as being limited to the
description in the following embodiment modes. Note that like reference
numerals are used to denote common portions and portions having a similar
function throughout the drawings for showing the embodiment modes, and
description thereof is omitted.

Embodiment Mode 1

[0038] In this embodiment mode, a switching element and a display device
including the switching element of the present invention will be
described with reference to the drawings.

[0039] First, the switching element will be described with reference to
FIGS. 1A to 1C. Note that FIG. 1A shows a top plan view of the switching
element and FIGS. 1B and 1C show cross-sectional views taken along line
A-B in FIG. 1A.

[0040] The switching element includes a first conductive film 101 and a
second conductive film 102 which are provided over a substrate 100, an
insulating film 103 which functions as a spacer layer, a gap 104, and a
third conductive film 105 (see FIGS. 1A and 1B). In the switching
element, the first conductive film 101 functions as a first electrode
(lower electrode), the third conductive film 105 functions as a third
electrode (upper electrode), and the second conductive film 102 functions
as a second electrode which is or is not in contact with the third
electrode.

[0041] The first conductive film 101 and the second conductive film 102
are provided over the substrate 100 so as to be adjacent to each other,
and the third conductive film 105 is provided over the first conductive
film 101 and the second conductive film 102. The insulating film 103 is
provided between the first conductive film 101 and the third conductive
film 105 at least in a region where the first conductive film 101 and the
third conductive film 105 overlap. A region where the insulating film 103
is not formed (opening 106) is provided in part or all of a portion where
the second conductive film 102 and the third conductive film 105 overlap.

[0042] The third conductive film 105 is provided across the second
conductive film 102 such that there is the gap 104 between the second
conductive film 102 and the third conductive film 105. In FIGS. 1A to 1C,
the third conductive film 105 is provided along the opening 106. In the
opening 106, the gap 104 is formed between the second conductive film 102
and the third conductive film 105. Further, in the opening 106, the gap
104 is also formed between the third conductive film 105 and the
insulating film 103 which functions as a spacer layer.

[0043] A switching element shown in FIGS. 1A to 1C can function as a
switch in the following manner. The insulating film 103 expands and
contracts so that contact or non contact operation is performed between
the second conductive film 102 and the third conductive film 105 (FIG.
1C). Potentials of the first conductive film 101 and the third conductive
film 105 are controlled to induce a charge of the first conductive film
101 and the third conductive film 105 and thus to cause compressive
stress to the insulating film 103 provided between the first conductive
film 101 and the third conductive film 105, so that the insulating film
103 can expand and contract.

[0044] For example, when the first conductive film 101 is set to be at a
constant potential and a given potential (for example, Vin)
different from the potential of the first conductive film 101 is applied
to the third conductive film 105, a charge is induced on the first
conductive film 101 and the third conductive film 105 which sandwich the
insulating film 103, and thus compressive stress is caused in the
insulating film 103, so the insulating film 103 contracts. As a result,
the second conductive film 102 and the third conductive film 105 come to
be in contact with each other, so the switching element is turned on
(contact state), and the second conductive film 102 can be set to be at a
potential the same as the potential Vin of the third conductive film
105.

[0045] In order to turn the switching element off (cause the switching
element to be in a noncontact state), a potential of the third conductive
film 105 is made approximately equal to a potential of the first
conductive film 101. At that time, a potential difference is not
generated between the first conductive film 101 and the third conductive
film 105 which sandwich the insulating film 103 and thus compressive
stress does not work on the insulating film 103, so that the second
conductive film 102 and the third conductive film 105 are physically
separated to be in a noncontact state. As a result, a leak current does
not flow.

[0046] When the switching element is off, a potential difference between
the third conductive film 105, which corresponds to an upper electrode,
and the first conductive film 101, which corresponds to a lower
electrode, is maintained constant so that malfunctioning of the switching
element can be prevented. Therefore, since the gap 104 between the second
conductive film 102 and the third conductive film 105 can be narrow, low
power consumption of the switching element can be achieved.

[0047] The insulating film 103 which functions as a spacer layer is
preferably formed using a material such as a piezoelectric element or an
electrostrictive vibrator, which contracts when a voltage is applied. For
example, the insulating film 103 can be formed using a piezoelectric
material such as a crystal (SiO2) film, a lead zirconate titanate
(PZT) film, a lithium niobate (LiNbO3) film, a barium titanate
(BaTiO3) film, a lead titanate (PbTiO3) film, a lead
metaniobate (PbNb2O6) film, or a zinc oxide (ZnO) film.
Alternatively, the insulating film 103 may be formed using a silicon
oxide (SiOx) film, a silicon oxynitride (SiOxNy, where
x>y) film, a silicon nitride (SiNx) film, a silicon nitride oxide
(SiNxOy, where x>y) film, or an aluminum nitride (AlNx)
film; or a multilayer film combining a piezoelectric material and any of
the above insulating films.

[0048] For example, in the structure shown in FIGS. 1A to 1C, a thickness
of the insulating film 103 can be 0.1 to 10 μm and the gap between the
second conductive film 102 and the third conductive film 105 can be 0.001
to 1 μm in the opening 106. The thickness and the gap of the
insulating film 103 may be set based on the amount of contraction of the
material of the insulating film 103 and the potential of the third
conductive film 105.

[0049] Thus, the insulating film provided between the conductive films
expands and contracts so that switching operation (contact or noncontact
operation) is performed; therefore, stress is not concentrated in a
portion of the switching element, so that damage to the element can be
prevented. Even in a case where contact and noncontact operations are
repeated, rebound of the conductive film 105 and deterioration of the
element can be prevented, and thus reliability of the switching element
can be improved. Since expansion and contraction in up and down
directions of the insulating film 103 are utilized, it is not necessary
for the gap 104 to be wide and the switching element can be miniaturized.
Further, when the switching element is off, a potential difference
between a conductive film, which corresponds to an upper electrode, and a
conductive film, which corresponds to a lower electrode, is set to be
constant, so that malfunctioning of the switching element can be
prevented even in the case where the switching element is driven with low
power consumption.

[0050] Further, the switching element described in this embodiment mode is
more efficient than a conventional mechanical switch in the following
respect.

[0051] A conventional switching element performs switching operation by
using electrostatic force between an upper electrode 131 and a lower
electrode 132 (see FIG. 18A). For example, when a given potential
(+Vg) is applied to the upper electrode 131, a negative charge is
induced on a surface of the lower electrode 132, and the upper electrode
131 and the lower electrode 132 come to be in contact with each other due
to electrostatic force, so the switching element is turned on (see FIG.
18B). Next, in order to turn the switching element off (to cause the
upper electrode 131 and the lower electrode 132 to not be in contact with
each other), the upper electrode 131 is set to be at a given potential
(for example, 0 V); however, there has been a problem in that in a case
where the potential of the lower electrode 132 is not constant and
varies, a charge is not discharged from the lower electrode 132, and
therefore, the upper electrode 131 and the lower electrode 132 remain in
contact with each other, so the switching element is not turned off (see
FIG. 18C).

[0052] Meanwhile, in a case where the switching element described in this
embodiment mode is off, a potential of the first conductive film 101
corresponding to a lower electrode is maintained at a given potential
(for example, Vcom) and a potential of the third conductive film 105
corresponding to an upper electrode is set to Vcom, and thus a
charge does not remain accumulated in an electrode. Therefore, there is
no possibility that problems such as those described above will be
caused, and therefore the switching element can be reliably turned off.

[0053] Note that the structure of the switching element described in this
embodiment mode is not limited to that in FIGS. 1A to 1C. Although a
structure is employed in which the gap 104 is also formed between the
first conductive film 101 and the second conductive film 102 which are
arranged over a substrate in FIGS. 1A to 1C, alternatively, a structure
may be employed in which the insulating film 103 covers gaps between the
first conductive film 101 and the second conductive film 102 and end
portions of the second conductive film 102 (FIG. 2A). Further, although a
structure in which the first conductive film 101 is provided so as to
partly surround the second conductive film 102 is shown in FIGS. 1A to
1C, a structure may be employed in which the first conductive film 101 is
provided only on one side of the second conductive film 102 (FIG. 2B).

[0054] Next, an example of a driving method of a display device which uses
the switching element shown in FIGS. 1A to 1C is described with reference
to FIGS. 3A and 3B. FIGS. 3A and 3B show schematic diagrams of a driver
circuit of the display device using the switching element shown in FIGS.
1A to 1C. Note that here, a structure is described in which a pixel
electrode is connected to the second conductive film 102 and a data line
is connected to a counter electrode provided so as to face the pixel
electrode. Further, while FIGS. 3A and 3B show a liquid crystal display
device provided with a liquid crystal material which is between the pixel
electrode and the counter electrode, the present invention is not limited
to this.

[0055] First, the potential of the first conductive film 101 which can
function as a common line is maintained at a common line potential
Vcom and a gate line potential Vg is applied to the third
conductive film 105 which can function as a gate line, so the switching
element is turned on (the second conductive film 102 and the third
conductive film 105 come to be in contact with each other). Note that a
data line potential Vsig is applied to the counter electrode.

[0056] In this case, the potential of the pixel electrode connected to the
second conductive film 102 is Vg, and thus a potential
(Vsig-Vg) is applied to liquid crystal materials provided
between the pixel electrode and the counter electrode. As a result,
alignment of the liquid crystal materials changes in accordance with the
potential (Vsig-Vg).

[0057] Next, the potential of the third conductive film 105 is made equal
to the common line potential Vcom and thus there is no (or little)
potential difference between the first conductive film 101 and the third
conductive film 105, so the switching element is turned off (the second
conductive film 102 and the third conductive film 105 cease to be in
contact with each other). When the switching element is off, the second
conductive film 102 and the pixel electrode are in a floating state;
therefore, the potential difference (Vsig-Vg) is maintained
until the switching element is next turned on.

[0058] By thus using the switching element, the alignment of the liquid
crystal materials of each pixel provided in the display device can be
controlled. Further, the potential (Vsig-Vg) applied to the
liquid crystal materials is varied by changing Vsig, and thus analog
gray-scale display is possible.

[0059] Note that the switching element and the display device including
the switching element, which are described above, can be implemented by
being combined with any one of the structures of the switching element or
the display device which are described in other embodiment modes in this
specification.

Embodiment Mode 2

[0060] In this embodiment mode, the structure of the display device
including the switching element described in the above embodiment mode
and a method for manufacturing the display device are described with
reference to the drawings.

[0061] First, the structure of the display device described in this
embodiment mode is described with reference to FIGS. 4 to 5B. Note that
FIG. 4 shows a top plan view of a pixel in the display device, FIG. 5A
shows a cross-sectional view taken along line A1-B1 in FIG. 4, and FIG.
5B shows a cross-sectional view taken along A2-B2 in FIG. 4.

[0062] The display device described in this embodiment mode includes a
conductive film 205 which can function as a gate line, a conductive film
201 which can function as a common line, a pixel electrode 211, a
conductive film 202 which is electrically connected to the pixel
electrode 211, a conductive film 221 which can function as a data line,
and a counter electrode 222 which is electrically connected to the
conductive film 221 (see FIG. 4). A switching element 230 includes the
conductive film 201, the conductive film 202, an insulating film 203, the
conductive film 205, and a gap 204 (see FIG. 5A). In the switching
element 230, the conductive film 201 corresponds to a first electrode
(lower electrode), the conductive film 202 corresponds to a second
electrode, the conductive film 205 corresponding to a third electrode
(upper electrode), and the insulating film 203 corresponds to a spacer
layer. As the structure of the switching element 230, any of the
structures described in the embodiment modes in this specification can be
employed.

[0063] The conductive film 201, the conductive film 202, and the pixel
electrode 211 are provided over the substrate 200, and the insulating
film 203 is provided so as to cover the conductive film 201 and part of
the conductive film 202. In addition, the conductive film 205 is provided
over the insulating film 203, and a region where the insulating film 203
is not formed (opening) is provided in part or all of a region where the
conductive film 202 and the conductive film 205 overlap.

[0064] An alignment film 216 is provided so as to cover the pixel
electrode 211, and a liquid crystal material 217 is provided between the
alignment film 216 and an alignment film 218 provided on the counter
substrate 220. Note that the counter substrate 220 is provided with the
conductive film 221 and the counter electrode 222, and the alignment film
218 is provided so as to cover the conductive film 221 and the counter
electrode 222 (see FIGS. 5A and 5B).

[0065] Note that the insulating film 203 may have a structure in which it
is provided between the conductive film 201 and the conductive film 205
in a region where at least the conductive film 201 and the conductive
film 205 overlap with each other, and it is also possible that the
insulating film 203 is not provided in a region other than this region
(see FIGS. 6A and 6B).

[0066] Next, the method for manufacturing the switching element and the
display device including the switching element is described with
reference to the drawings. Note that in a following description, FIGS. 7A
to 8D each show a cross-sectional view taken along line A1-B1 in FIG. 4,
and FIGS. 9A to 11B each show a cross-sectional view taken along A2-B2 in
FIG. 4.

[0067] First, the pixel electrode 211 is selectively formed over the
substrate 200 (see FIG. 9A).

[0068] As the substrate 200, a light-transmitting substrate such as a
glass substrate, a quartz substrate, or a plastic substrate can be used.
For the pixel electrode 211, an indium tin oxide (ITO) film in which
indium oxide is added with tin oxide, an indium tin silicon oxide film in
which indium tin oxide (ITO) is added with silicon oxide, an indium zinc
oxide (IZO) film in which indium oxide is added with zinc oxide, a zinc
oxide film, a tin oxide film, or the like can be used.

[0069] Next, the conductive film 201 and the conductive film 202 are
selectively formed over the substrate 200 (see FIGS. 7A and 9B). The
conductive film 202 is provided so as to be electrically connected to the
pixel electrode 211. The conductive film 201 and the conductive film 202
may be provided in the following manner. A conductive film is formed on
an entire surface by a CVD method, a sputtering method, or the like and
then is selectively etched. Alternatively, the conductive film 201 and
the conductive film 202 may be selectively provided by a droplet
discharging method.

[0070] Each of the conductive film 201 and the conductive film 202 is
formed to have a single-layer structure or a stacked-layer structure
using an element selected from aluminum (Al), tungsten (W), titanium
(Ti), tantalum (Ta), molybdenum (Mo), nickel (Ni), platinum (Pt), copper
(Cu), gold (Au), silver (Ag), manganese (Mn), neodymium (Nd), carbon (C),
and silicon (Si), or an alloy material or a compound material containing
any of the above elements as its main component.

[0071] Note that while a case where the pixel electrode 211 is formed and
then the conductive film 202 is provided so as to be electrically
connected to the pixel electrode 211 is described here, it is also
allowed that the conductive film 201 and the conductive film 202 are
formed and then the pixel electrode 211 is formed.

[0072] Next, the insulating film 203 is formed so as to cover the
conductive film 201, the conductive film 202, and the pixel electrode 211
(see FIGS. 7B and 9C). Functioning as a spacer layer in a switching
element to be completed later, the insulating film 203 is preferably
formed using a material such as a piezoelectric element which contracts
by voltage application. Therefore, the insulating film 203 may be formed
using a crystal (SiO2) film, a lead zirconate titanate (PZT) film, a
lithium niobate (LiNbO3) film, a barium titanate (BaTiO3) film,
a lead titanate (PbTiO3) film, a lead metaniobate
(PbNb2O6) film, a zinc oxide (ZnO) film, or the like.
Alternatively, the insulating film 203 may be formed using a silicon
oxide (SiOx) film, a silicon oxynitride (SiOxNy, where
x>y) film, a silicon nitride (SiNx) film, a silicon nitride oxide
(SiNxOy, where x>y) film, or an aluminum nitride (AlNx)
film; or a multilayer film combining a piezoelectric material and any of
the above insulating films.

[0073] Next, part or all of the insulating film 203 formed over the
conductive film 202 is selectively etched so that an opening 206 is
formed (see FIGS. 7C and 9D). The opening 206 is formed at least in part
or all of a portion where the conductive film 202 and the conductive film
205 to be formed later, which can function as a gate line, overlap with
each other.

[0074] Next, a sacrificial layer 212 is formed over the insulating film
203 and the conductive film 202 in the opening 206 (see FIGS. 7D and 9E).
The sacrificial layer refers to a layer to be removed in a later step,
and by removing the sacrifice layer, a gap is formed. The sacrificial
layer 212 can be formed of a material containing a metal element, a metal
compound, silicon, a silicon oxide, or a silicon nitride. Here, for the
sacrificial layer 212, a zinc oxide (ZnOx) or a zinc sulfide (ZnS) is
formed by a sputtering method.

[0075] Next, the sacrificial layer 212 is selectively etched so that the
sacrificial layer which is over the opening 206 and the insulating film
203 in the vicinity thereof remains. Here, an example in which etching is
performed so that the sacrificial layer 213 remains is described (see
FIGS. 7E and 10A).

[0076] Next, the conductive film 214 is formed over the remaining
sacrificial layer 213 and the insulating film 203 (see FIGS. 8A and 10B).
The conductive film 214 is formed by a CVD method, a sputtering method,
or the like to have a single-layer structure or a stacked-layer structure
using an element selected from aluminum (Al), tungsten (W), titanium
(Ti), tantalum (Ta), molybdenum (Mo), nickel (Ni), platinum (Pt), copper
(Cu), gold (Au), silver (Ag), manganese (Mn), neodymium (Nd), carbon (C),
and silicon (Si), or an alloy material or a compound material containing
any of the above elements as its main component. Here, aluminum is used.

[0077] Next, the conductive film 214 is selectively removed by being
etched. Here, an example where etching is performed so that the
conductive film 205 remains is described (see FIG. 10C). Note that while
an example where the conductive film 205 is provided over the opening 206
and the insulating film 203 in the vicinity thereof is described in this
embodiment mode, it is also allowed that the conductive film 205 is not
provided over the insulating film 203.

[0078] Next, the sacrificial layer 213 is selectively removed so that the
gap 204 is formed in a portion where the sacrificial layer 213 had been
provided, and thus the switching element 230 is obtained (see FIGS. 8B
and 10D). Note that the sacrificial layer 213 may be removed at the same
time that the conductive film 214 is removed. For example, in a case
where a zinc oxide (ZnOx) is used for the sacrificial layer 213 and
aluminum is used for the conductive film 214, wet etching is performed
using an etching solution (for example, a mixed acid containing a nitric
acid (HNO3), an acetic acid (CH3COOH), and a phosphoric acid
(H3PO4)). At this time, an etching rate of ZnOx with respect to
a mixed acid containing a nitric acid (HNO3), an acetic acid
(CH3COOH), and a phosphoric acid (H3PO4) is extremely
faster than (approximately two hundred times as fast as) that of
aluminum; therefore, ZnOx can be removed at the same time that aluminum
is etched.

[0079] Next, the insulating film 203 provided over the pixel electrode 211
is selectively removed so that an opening 215 is formed (see FIG. 10E).
Note that the opening 215 may be formed at the same time that the opening
206 is formed (FIGS. 7C and 9D).

[0080] Next, an alignment film 216 is formed so as to cover the pixel
electrode 211 and the like (see FIGS. 8C and 11A).

[0081] Next, the counter substrate 220, which is provided in advance with
the conductive film 221, the counter electrode 222, and the alignment
film 218, is attached to the substrate 200 with a space of several μm
interposed therebetween and then a liquid crystal material 217 is
injected between the substrate 200 and the counter substrate 220, so a
display device can be obtained (see FIGS. 8D and 11B). Note that the
counter substrate 220 which is provided with a light-shielding film, a
color filter, a spacer, and the like may be used. Alternatively, another
manufacturing method may be employed in which the alignment film 216 is
formed, a spacer is formed and the liquid crystal material 217 is formed
by liquid crystal dropping, and then the counter substrate 220 is
attached to the substrate 200.

[0082] Hereinafter, a method for manufacturing the counter substrate 220
which is provided with the conductive film 221, the counter electrode
222, and the like is described with reference to FIGS. 12A to 12D.

[0083] First, a plurality of the counter electrodes 222 are formed over
the counter substrate 220 (see FIG. 12A). As the counter substrate 220, a
light-transmitting substrate such as a glass substrate, a quartz
substrate, or a plastic substrate can be used. For the counter electrode
222, an indium tin oxide (ITO) film in which indium oxide is added with
tin oxide, an indium tin silicon oxide film in which indium tin oxide
(ITO) is added with silicon oxide, an indium zinc oxide (IZO) film in
which indium oxide is added with zinc oxide, a zinc oxide film, a tin
oxide film, or the like can be used.

[0084] Next, the conductive film 221 is selectively formed over the
counter substrate 220. The conductive film 221 is provided so as to be
electrically connected to the counter electrode 222 (see FIG. 12B). The
conductive film 221 can function as a data line in the display device.

[0085] The conductive film 221 is formed by a CVD method, a sputtering
method, or the like to have a single-layer structure or a stacked-layer
structure using an element selected from aluminum (Al), tungsten (W),
titanium (Ti), tantalum (Ta), molybdenum (Mo), nickel (Ni), platinum
(Pt), copper (Cu), gold (Au), silver (Ag), manganese (Mn), neodymium
(Nd), carbon (C), and silicon (Si), or an alloy material or a compound
material containing any of the above elements as its main component.
Alternatively, a droplet discharging method may be employed.

[0086] Note that while a case where the counter electrode 222 is formed
and then the conductive film 221 is provided so as to be electrically
connected to the counter electrode 222 is described here, it is also
allowed that the conductive film 221 is formed and then the counter
electrode 222 is formed.

[0087] Next, the alignment film 218 is formed so as to cover the counter
electrode 222 and the conductive film 221 (see FIG. 12C). Further, a
color filter 223 may be provided so as to overlap with the counter
electrode 222 (see FIG. 12D).

[0088] Through the above process, the display device can be obtained.

[0089] With the manufacturing method described in this embodiment mode, a
step of crystallizing a semiconductor film or a step of introducing
impurity elements is not performed; therefore, a manufacturing process
can be simplified as compared to a manufacturing process of a display
device using thin film transistors (TFTs). As a result, the display
device can be manufactured at low cost.

[0090] Note that the switching element and the display device including
the switching element, which are described above, can be implemented by
being combined with any one of the structures of the switching element or
the display device which are described in other embodiment modes in this
specification.

Embodiment Mode 3

[0091] In this embodiment mode, a different method for manufacturing a
switching element from the above embodiment mode is described with
reference to the drawings. In specific, a case of forming a gap without
using a sacrificial layer in the method for manufacturing a switching
element is described.

[0092] In this embodiment mode, a gap is formed by applying volume
contraction due to cohesion. Hereinafter, description is made with
reference to FIGS. 13A to 14B.

[0093] First, the first conductive film 101 and the second conductive film
102 are selectively formed over the substrate 100 and then the insulating
film 103 which functions as a spacer layer is formed so as to cover the
first conductive film 101 and the second conductive film 102 (see FIG.
13A).

[0094] Each of the first conductive film 101 and the second conductive
film 102 is formed to have a single-layer structure or a stacked-layer
structure using an element selected from aluminum (Al), tungsten (W),
titanium (Ti), tantalum (Ta), molybdenum (Mo), nickel (Ni), platinum
(Pt), copper (Cu), gold (Au), silver (Ag), manganese (Mn), neodymium
(Nd), carbon (C), and silicon (Si), or an alloy material or a compound
material containing any of the above elements as its main component. Note
that here, titanium (Ti) is formed for the first conductive film 101 and
the second conductive film 102.

[0095] The insulating film 103 is formed to have a single-layer structure
of a silicon oxide film, a silicon oxynitride film, a silicon nitride
film, a silicon nitride oxide film, an aluminum nitride (AlNx) film, a
lead zirconate titanate (PZT) film, a lithium niobate (LiNbO3) film,
a barium titanate (BaTiO3) film, a lead titanate (PbTiO3) film,
a lead metaniobate (PbNb2O6) film, a zinc oxide (ZnO) film, or
the like or a multilayer structure combining any of the above. Here, a
silicon nitride (SiNx) film is formed as the insulating film 103.

[0096] Then, the insulating film 103 is selectively etched so that at
least part of the second conductive film 102 is exposed and thus the
opening 106 is formed (see FIG. 13B).

[0097] Then, a conductor 121 is selectively formed in the opening (see
FIG. 13c). The conductor 121 can be formed using conductive particles of
silver (Ag), copper (Cu), nickel (Ni), or the like. Note that here,
nanoparticles of silver (Ag) are selectively formed in the opening 106 by
a droplet discharging method.

[0098] Then, the third conductive film 105 is selectively formed so as to
cover the conductor 121 formed in the opening 106 (see FIG. 14A). The
third conductive film 105 is formed to have a single-layer structure or a
stacked-layer structure using an element selected from aluminum (Al),
tungsten (W), titanium (Ti), tantalum (Ta), molybdenum (Mo), nickel (Ni),
platinum (Pt), copper (Cu), gold (Au), silver (Ag), manganese (Mn),
neodymium (Nd), carbon (C), and silicon (Si), or an alloy material or a
compound material containing any of the above elements as its main
component. Note that here, molybdenum (Mo) is formed for the third
conductive film 105.

[0099] Then, the conductor 121 is aggregated by heat treatment, so that
the gap 104 is formed between the conductor 121 and the third conductive
film 105 (see FIG. 14B). Here, heat treatment is performed at 200 to
600° C. in a nitrogen atmosphere to remove an organic film
covering nanoparticles of Ag, so that Ag aggregates. In this case, having
high adhesion to titanium (Ti) of the first conductive film 101,
nanoparticles of Ag have low adhesion to molybdenum (Mo) of the third
conductive film 105; therefore, when Ag aggregates and the volume thereof
is reduced, the gap 104 is formed between the conductor 121 and the third
conductive film 105. Thus, the switching element can be manufactured.

[0100] With the manufacturing method described in this embodiment mode, a
gap can be formed without using a sacrificial layer. Therefore, a
manufacturing process can be simplified.

[0101] Note that the method for manufacturing a switching element, which
is described in this embodiment mode, can be implemented by being
combined with any one of the methods for manufacturing a switching
element or a display device including a switching element which are
described in other embodiment modes in this specification.

Embodiment Mode 4

[0102] In this embodiment mode, the structure of a different switching
element from the above embodiment mode is described with reference to the
drawings.

[0103] The switching element described in this embodiment mode includes
the first conductive film 101, the second conductive film 102, and a
fourth conductive film 151 which are provided over the substrate 100, the
insulating film 103, the gap 104, the third conductive film 105, and a
fifth conductive film 152 (see FIGS. 15A and 15B). The structure shown in
FIGS. 15A to 15C differs from the structure shown in FIGS. 1A to 1C in
that the fourth conductive film 151 which can function as a data line is
arranged in parallel with the second conductive film 102, and the fifth
conductive film 152 which is electrically independent is provided so as
to overlap with the second conductive film 102 and the fourth conductive
film 151.

[0104] In specific, the first conductive film 101, the second conductive
film 102, and the fourth conductive film 151 are arranged over the
substrate 100, the fifth conductive film 152 is provided at least over
the second conductive film 102 and the fourth conductive film 151, and
the third conductive film 105 is provided at least over the first
conductive film 101. The insulating film 103 is provided at least between
the first conductive film 101 and the third conductive film 105, and the
opening 106 is formed in part or all of a portion where the fifth
conductive film 152 overlaps with the second conductive film 102 and the
fourth conductive film 151.

[0105] Further, in the opening 106, the gap 104 is formed between the
second conductive film 102 and the fifth conductive film 152 and between
the fourth conductive film 151 and the fifth conductive film 152.

[0106] The switching element shown in FIGS. 15A to 15C functions as a
switch in the following manner. The insulating film 103 expands and
contracts so that contact or non contact operation is performed between
the fifth conductive film 152, and the second conductive film 102 and the
fourth conductive film 151 (FIG. 15c).

[0107] For example, in a case where the potential of the first conductive
film 101 is Vcom, the potential of the fourth conductive film 151 is
Vsig, and a potential Vin (Vin≠Vcom) is applied
to the third conductive film 105, a charge is induced on the first
conductive film 101 and the third conductive film 105 which sandwich the
insulating film 103 and thus compressive stress is caused to the
insulating film 103 so that the insulating film 103 contracts. As a
result, the fifth conductive film 152, which is provided so as to be
electrically independent, comes in contact with the second conductive
film 102 and the fourth conductive film 151, and the potential Vsig
of the fourth conductive film 151 is applied to the second conductive
film 102.

[0108] Thus, the insulating film provided between the conductive films
expands and contracts so that switching operation (contact or noncontact
operation) is performed; therefore, stress is not concentrated in a
portion of the switching element, and damage to the element can be
prevented. Even in a case where contact and noncontact operations are
repeated, rebound of the fifth conductive film 152 and deterioration of
the element can be prevented, and thus reliability of the switching
element can be improved. Since expansion and contraction in up and down
directions of the insulating film 103 are utilized, it is not necessary
for the gap 104 to be wide and the switching element can be miniaturized.
Further, when the switching element is off, a potential difference
between a conductive film, which corresponds to an upper electrode, and a
conductive film, which corresponds to a lower electrode, is set to be
approximately constant, so that malfunctioning of the switching element
can be prevented even in the case where the switching element is driven
with low power consumption.

[0109] Next, an example of a method for driving a display device using the
switching element shown in FIGS. 15A to 15C is described below with
reference to FIG. 16. FIG. 16 shows a schematic diagram of a driver
circuit of the display device using the switching element shown in FIGS.
15A to 15C. Here, a case is described where a pixel electrode is
connected to the second conductive film 102 and a counter electrode
provided so as to face the pixel electrode holds a constant potential
(Vcom).

[0110] First, the first conductive film 101 which can function as a common
line holds a common line potential Vcom and a gate line potential
Vg is applied to the third conductive film 105 which can function as
a gate line, so the switching element is turned on (the fifth conductive
film 152 comes to be in contact with the second conductive film 102 and
the fourth conductive film 151). Note that the data line potential
Vsig is applied to the fourth conductive film 151 which can function
as a data line.

[0111] In this case, Vsig is applied to the pixel electrode connected
to the second conductive film 102 from the fourth conductive film 151
through the fifth conductive film 152, and thus a potential of
(Vcom-Vsig) is applied to a display element provided between
the pixel electrode and the counter electrode. As a result, in a case of
providing a liquid crystal material for a display element, the alignment
of liquid crystals varies in accordance with the potential of
(Vcom-Vsig).

[0112] Next, the potential of the third conductive film 105 is made equal
to the common line potential Vcom, so the switching element is
turned off (the fifth conductive film 152 cease to be in contact with the
second conductive film 102 and the fourth conductive film 151). When the
switching element is off, the second conductive film 102 and the pixel
electrode are in a floating state; therefore, the potential difference of
(Vcom-Vsig) is maintained until the switching element is next
turned on.

[0113] Thus, by using the switching element, the alignment of the liquid
crystal materials of each pixel provided in the display device can be
controlled. Further, the potential of (Vcom-Vsig) applied to
the liquid crystal material is varied by changing Vsig and thus
analog gray-scale display is possible.

[0114] Note that although a case of using a liquid crystal material for
the display element in the structure of the display device described and
shown in Embodiment Modes 1 to 3 and FIG. 16 is described, a display
device to which the switching element described in this specification can
be applied is not limited to a liquid crystal display device. For
example, the switching element can be applied to a display device using
an organic EL material or an inorganic EL material as a display element.

[0115] In the case of using an inorganic EL material for the display
element, a driving voltage is comparatively high (100 to 200 V);
therefore, there is a problem such as breakage in a case of applying a
thin film transistor (TFT) as the switching element. However, by applying
the switching element described in this specification, the switching
element is prevented from being broken and a highly reliable display
device can be provided even in the case where a driving voltage is high.

[0116] Further, the switching element described in this specification can
be applied to a memory element such as a DRAM (dynamic random access
memory) as well as a display device. In this case, a capacitor is
provided instead of a pixel electrode for the second conductive film 102
in the structure described in any of Embodiment Modes 1 to 4 and this
embodiment mode, so that a memory element can be formed. Further, a
charge accumulation layer which can hold a charge is connected to the
second conductive film 102, and thus the second conductive film 102 can
be applied as a memory element depending on whether a charge is in an
accumulation state or not.

[0117] Note that the switching element and the display device including
the switching element, which are described above, can be implemented by
being combined with any one of the structures of the switching element or
the display device which are described in other embodiment modes in this
specification.

Embodiment Mode 5

[0118] The display device of the present invention can be applied to
various electronic appliances, specifically, a display portion of
electronic appliances. The electronic appliances include cameras such as
a video camera and a digital camera, a goggle-type display, a navigation
system, an audio reproducing device (a car audio component stereo, an
audio component stereo, or the like), a computer, a game machine, a
portable information terminal (a mobile computer, a mobile phone, a
mobile game machine, an electronic book, or the like), an image
reproducing device having a recording medium (specifically, a device for
reproducing a recording medium such as a digital versatile disc (DVD) and
having a display device for displaying the reproduced image) and the
like.

[0119]FIG. 17A shows a TV set which includes a housing 301, a supporting
base 302, a display portion 303, a speaker portion 304, a video inputting
terminal 305, and the like. The switching element described in any of the
above embodiment modes can be used to drive the display portion 303. Note
that the TV set includes all light-emitting devices for displaying
information for a personal computer, for receiving television
broadcasting, for displaying an advertisement, and the like.

[0120]FIG. 17B shows a camera which includes a main body 311, a display
portion 312, an image receiving portion 313, operation keys 314, an
external connection port 315, a shutter button 316, and the like. The
switching element described in any of the above embodiment modes can be
used for display portion 312.

[0121]FIG. 17c shows a computer which includes a main body 321, a housing
322, a display portion 323, a keyboard 324, an external connection port
325, a pointing device 326, and the like. The switching element described
in any of the above embodiment modes can be used to drive the display
portion 323.

[0122]FIG. 17D shows a mobile computer which includes a main body 331, a
display portion 332, a switch 333, operation keys 334, an infrared port
335, and the like. The switching element described in any of the above
embodiment modes can be used to drive the display portion 332.

[0123]FIG. 17E shows a portable image reproducing device having a
recording medium (in specific, a DVD reproducing device), which includes
a main body 341, a housing 342, a display portion A 343, a display
portion B 344, a recording medium (DVD or the like) reading portion 345,
an operation key 346, a speaker portion 347, and the like. The display
portion A 343 mainly displays image data and the display portion B 344
mainly displays text data. The switching element described in any of the
above embodiment modes can be used to drive the display portions A 343
and B 344.

[0124]FIG. 17F shows a goggle-type display which includes a main body
351, a display portion 352, and an arm portion 353. The switching element
described in any of the above embodiment modes can be used to drive the
display portion 352.

[0125]FIG. 17G shows a video camera which includes a main body 361, a
display portion 362, a housing 363, an external connection port 364, a
remote control receiving portion 365, an image receiving portion 366, a
battery 367, an audio inputting portion 368, operation keys 369, and the
like. The switching element described in any of the above embodiment
modes can be used to drive the display portion 362.

[0126]FIG. 17H shows a mobile phone which includes a main body 371, a
housing 372, a display portion 373, an audio input portion 374, an audio
output portion 375, operation keys 376, an external connection port 377,
an antenna 378, and the like. The switching element described in any of
the above embodiment mode can be used to drive the display portion 373.

[0127] Thus, the present invention can be applied to various electronic
appliances each including a switch.

[0128] This application is based on Japanese Patent Application serial no.
2007-078558 filed with Japan Patent Office on Mar. 26, 2007, the entire
contents of which are hereby incorporated by reference.